Plugin modules must not be confused with external ARexx scripts, historically also called "plugins". A plugin module is a shared library with a specific interface. Plugin modules are allowed to render directly into the AWeb window, either as a standalone object, or as an embedded object in a HTML document.
The API (Application Programmers Interface) is freely available from http://www.xs4all.nl/~yrozijn/aweb/api.html. The archive contains all documentation, C header files, and an example plugin module with source (written in C).